THE VISUAL PIGMENTS 



to allow a flow of water through it from a thermostat, thus main- 

 taining the temperature constant to within 1°C. The cyhndrical 

 optical cell (internal diameter 1-0 cm, length 0-5 cm) was of fused 

 quartz with plane parallel faces. It fitted coaxially into a cavity in 

 the holder, the aperture of which was of the same diameter as the 



Fig. 3.2. Apparatus for studying the bleaching kinetics of photo- 

 sensitive solutions by the method of photometric curves. 

 {Dartnall, Goodeve and Lythgoe, 1936) 



cell. The beam of Hght from the exit slit was sHghtly divergent but 

 the spreading over the short optical path through the visual purple 

 solutions could be neglected. 



After passing through the visual purple solution, the monochro- 

 matic beam fell on a photoelectric cell, Q, which was connected to a 

 critically-damped galvanometer, G^. Between the cell holder and the 



66 



